Jump to content
Ernie Christiansen

1975 DT 175 Headlight not working - Please Help

Recommended Posts

I have a 1975 Yamaha DT 175 and the Head light is not working... the bike starts up fine with or without a battery in it and the horn and all other lights (turn signals and breaklights) work just fine with it running or not running...

At the head light bucket I am getting practically nothing (like 1.2 volts) any ideas where to start on this I cant ride this baby with out a head light and I am terribly frustrated right now

Share this post


Link to post
Share on other sites

I have a 1975 Yamaha DT 175 and the Head light is not working... the bike starts up fine with or without a battery in it and the horn and all other lights (turn signals and breaklights) work just fine with it running or not running...

At the head light bucket I am getting practically nothing (like 1.2 volts) any ideas where to start on this I cant ride this baby with out a head light and I am terribly frustrated right now

The horn and other lights run off the battery while the head and tail lights and instrument lights run off AC current directly from the lighting coil and flywheel. With ALL AC lights in operation, the circuit will be balanced and the voltage will be the same at all points at a given RPM. So you need to have your tester on AC. If the volts are too high or too low, check for bad connections, damaged wires or burned out bulbs on the AC circuit.

Then.....If no joy there check the resistance in the windings of the lighting coil.

Hang in there, things have a way of working out. Don't work on it frustrated. mTake the dog for a walk and come back to it

Share this post


Link to post
Share on other sites

I have a 1975 Yamaha DT 175 and the Head light is not working... the bike starts up fine with or without a battery in it and the horn and all other lights (turn signals and breaklights) work just fine with it running or not running...

At the head light bucket I am getting practically nothing (like 1.2 volts) any ideas where to start on this I cant ride this baby with out a head light and I am terribly frustrated right now

Don't forget to check the handle switch. Could be corroded or broken. If it is the headlight connector will not have any power to it.

Share this post


Link to post
Share on other sites

I keep finding myself saying this: i have little experience but my own with a 74 dt175 but.

my headlamp grounds thru the speedo cable. on its way it passes thru the speedo lamp bulb.

headlight + :to: headlamp :to: headlamp - (black wire) :to: instrument bulb on speedo :to: frame ground via speedo cable.

it sounds strange but..... if i remove the bulb assembly from the speedo. the ground is interrupted and no light. ditto if i remove the speedo cable.

try running an extra ground wire to frame at the headlamp.

(or better yet listen to the people here that know what they are talking about)

Share this post


Link to post
Share on other sites

The horn and other lights run off the battery while the head and tail lights and instrument lights run off AC current directly from the lighting coil and flywheel. With ALL AC lights in operation, the circuit will be balanced and the voltage will be the same at all points at a given RPM. So you need to have your tester on AC. If the volts are too high or too low, check for bad connections, damaged wires or burned out bulbs on the AC circuit.

Then.....If no joy there check the resistance in the windings of the lighting coil.

Hang in there, things have a way of working out. Don't work on it frustrated. mTake the dog for a walk and come back to it

Thanks for the Help on this... Here is the current Status... I have all the lights working including the Headlamp.... but now It seems my volatage is too high cause I am blowing bulbs... the bike stem is stamped as a 1975 175DT but the wiring diagram for that bike shows no volatage regulator and their is DEFINITELY a Voltage regulator on the air box of this bike... At 6000 to 7000 Rpm I am seeing 10 to 12 volts at the battery

The 1976 Had a voltage regulator on it so I am guessing my bike is somewhere in between... So here is the question... How do you test a voltage regulator and how does it working sitting with in the System (mine has just a yellow wire in and a black wire to ground)... also for a new one Yamaha wants $310 and there is no way I can afford that... are there any other options!?!?!??!

Whew... Long Winded I know!!!! but ANY help will be greatly appreaciated

Share this post


Link to post
Share on other sites

Thanks for the Help on this... Here is the current Status... I have all the lights working including the Headlamp.... but now It seems my volatage is too high cause I am blowing bulbs... the bike stem is stamped as a 1975 175DT but the wiring diagram for that bike shows no volatage regulator and their is DEFINITELY a Voltage regulator on the air box of this bike... At 6000 to 7000 Rpm I am seeing 10 to 12 volts at the battery

The 1976 Had a voltage regulator on it so I am guessing my bike is somewhere in between... So here is the question... How do you test a voltage regulator and how does it working sitting with in the System (mine has just a yellow wire in and a black wire to ground)... also for a new one Yamaha wants $310 and there is no way I can afford that... are there any other options!?!?!??!

Whew... Long Winded I know!!!! but ANY help will be greatly appreaciated

The voltage regulator is a Zener diode that holds a voltage constant by leaking current to earth via resistance. It cannot be checked without proper test equipment except by substitution.

Three hundred beer tokens is a bit rich, they do come up, i bagged one off ebay for mine for £30.

You can get generic 6 and 12 volt regulators for nowhere near that price it just depends if you want originality or just a working bike. It would be better if you could try a working item to prove the problem before laying out on a new one as they are pretty reliable.

Share this post


Link to post
Share on other sites

The voltage regulator is a Zener diode that holds a voltage constant by leaking current to earth via resistance.

It can be tested, the best way is to use two multimeters, connect one on series and set this to read DC current, connect the other in parallel, ie between the wire to the reg and to chassis and set to read DC voltage. Start the engine and increase the revs, as the revs increase the voltage rises above the nominal battery voltage you will see that the meter reading the current will start to read the current that is shunted back to chassis ( or battery negative) if your reg is working correctly.

You could of course use one meter, first do the voltage versus revs and take note of the results, then connect for current in series, you will know what voltage is at any given revs and can see the current on the meter.

Share this post


Link to post
Share on other sites

Thanks for the Help on this... Here is the current Status... I have all the lights working including the Headlamp.... but now It seems my volatage is too high cause I am blowing bulbs... the bike stem is stamped as a 1975 175DT but the wiring diagram for that bike shows no volatage regulator and their is DEFINITELY a Voltage regulator on the air box of this bike... At 6000 to 7000 Rpm I am seeing 10 to 12 volts at the battery

The 1976 Had a voltage regulator on it so I am guessing my bike is somewhere in between... So here is the question... How do you test a voltage regulator and how does it working sitting with in the System (mine has just a yellow wire in and a black wire to ground)... also for a new one Yamaha wants $310 and there is no way I can afford that... are there any other options!?!?!??!

Whew... Long Winded I know!!!! but ANY help will be greatly appreaciated

One point to note the air box is rubber mounted, at least it is on my Dt175 1g1 so on my bikr the regulator is earthed to the air box then the air box is also earthed,

Share this post


Link to post
Share on other sites

I am having the same problem with my 74 dt175.

the headlight works, but if i fry my battery, then there is no buffer and all the volts (up to 27) go to the headlight.

my charger output with a new battery is 5v at idle and 8v at redline. if i fry a battery, the headlight voltage will spike to 18 at idle and 27 at full rev. I need to keep the charging output limited to 6v.

I have 2 diode rectifiers on hand, and they both work the same, so i believe that the possibility that they are both malfunctioning the same way is pretty slim.

i need a voltage regulator at the point before the charging circuit enters the battery.

the cheapest one i can find is for a volkswagen 6v. it will handel up to 30 amps but its $40 us.

can anybody point me towards a cheaper one?

i will admit that 40 bucks is pretty cheap considering that the batteries that im frying are 20 bucks each but....

if i could find one that is less i would be happier. im not at all concerned with keeping yanaha stock. i just want it to run and not fry batteries anymore. as a matter of fact i have substituted the standard "wet cell" with fill holes for a sealed cell 6v battery at 5a capacity (the stock "wet cell" has a 2.5a capacity.)

I tried radio shack and they are a bunch of idiots. all they do there now is sell cell phones, gps and batteries. nobody there knows about "electronics" they cant even answer questions i have about ohm load on speakers or resistor loads for other apps.

my bike is old, run down, and flurry to a host of backwoods mechanics. returning it to its full stock glory is out of the question. but it runs well and i want to keep it that way.

if anybody has one, i will buy it. or if you out there can find me one i will gladly buy it thru you, or order it from the place you suggest.

thanks.

-little dave

Share this post


Link to post
Share on other sites

Way out of my depth here so the posts have informed me a great deal....Ta!

Is the rectifier on the 250a (74) the bit that is the "regulator"? Seems the terms interchange a bit. Anyway, I ordered a new rectifier at about $30AU.

Share this post


Link to post
Share on other sites

Way out of my depth here so the posts have informed me a great deal....Ta!

Is the rectifier on the 250a (74) the bit that is the "regulator"? Seems the terms interchange a bit. Anyway, I ordered a new rectifier at about $30AU.

nah, the rectifier, or rectifier diode. converts a/c output from the magneto to d/c output usually 6 or 12 volts(ish).

but as it has no earth mount, it is not capable of regulating the voltage to a certain limit. so it can vary from 5-8v depending on engine speed.

(atleast thats the way i understand it)

Share this post


Link to post
Share on other sites

Hi Little Dave,

Yeah I don't understand the workings of it but in my book it says if AC voltage still high after the lighting circuit checks out ok, the fault might be with the DC charging circuit b/c they share the same source coil. The way it reads to me is if the voltage is low due to a faulty battery, rectifier or connection, in the charging circuit, the voltage will then be too high in the lighting circuit. That's for the 250 which I'm assuming is the same for the 175. There's no mention of a regulator only the rectifier.

Anyways, after all this talk of lights and stuff I checked mine and errrrrr, she no look good. I've only had my bike on the road for a couple of months and don't ride much and then only during the day. Maybe I'm being punished by God or something. I think I'll just make myself a cup of tea, go back through the posts, then sneak out to the shed.....

Share this post


Link to post
Share on other sites

Hey,

I came across a couple of bulletins from Yamaha relating to blowing headlights and voltage regulators for pre-76 magneto bikes:

BulletinM6-058.gif

BulletinM5-084.gif

Share this post


Link to post
Share on other sites

I am having the same problem with my 74 dt175.

the headlight works, but if i fry my battery, then there is no buffer and all the volts (up to 27) go to the headlight.

my charger output with a new battery is 5v at idle and 8v at redline. if i fry a battery, the headlight voltage will spike to 18 at idle and 27 at full rev. I need to keep the charging output limited to 6v.

I have 2 diode rectifiers on hand, and they both work the same, so i believe that the possibility that they are both malfunctioning the same way is pretty slim.

-little dave

OK More Statuses.... First off... LOL.... My Bike is officially a 76

I bought a NEW rectifier and a "New" (used) voltage regulator off of e-bay.... and a brand new battery.... and I still have the problem.... Little Dave describes the problem perfectly.... When I test across the battery is does ok because the battery is buffering it and getting the crap kicked out of it... when if frys the whole system blows bulbs... When I test inline in front of the battery (which should be the charging circuit... I am assumming) I see voltage that is off the chart according to the manuals little table....

So here is the basics for my system

    Brand New Battery
    Brand New Recitifier
    Replaced Regulator (Absolutely no differance)

Now what do I do/test/Pray for in order to keep from frying batteries and/or blowing blubs

The manual is of no help because it just says

If the Charging System does not meet specifications, Check all wiring, charging coil, connections, main switch, and rectifier..

Check for WHAT!?!?!?!? That they exist!?!?!? (Cause I did that and the wires are there and the connections are there... and LOL... I am assuming the charging coil is working cause its charging the shiet muslim out of my battery!!! Check how?!?!?! Its a high volatage issue.... any more help would be greatly appreciated

Thanks a ton guys!!!

Ernie

Share this post


Link to post
Share on other sites

OK More Statuses.... First off... LOL.... My Bike is officially a 76

I bought a NEW rectifier and a "New" (used) voltage regulator off of e-bay.... and a brand new battery.... and I still have the problem.... Little Dave describes the problem perfectly.... When I test across the battery is does ok because the battery is buffering it and getting the crap kicked out of it... when if frys the whole system blows bulbs... When I test inline in front of the battery (which should be the charging circuit... I am assumming) I see voltage that is off the chart according to the manuals little table....

So here is the basics for my system

    Brand New Battery
    Brand New Recitifier
    Replaced Regulator (Absolutely no differance)

Now what do I do/test/Pray for in order to keep from frying batteries and/or blowing blubs

The manual is of no help because it just says

Check for WHAT!?!?!?!? That they exist!?!?!? (Cause I did that and the wires are there and the connections are there... and LOL... I am assuming the charging coil is working cause its charging the shiet muslim out of my battery!!! Check how?!?!?! Its a high volatage issue.... any more help would be greatly appreciated

Thanks a ton guys!!!

Ernie

Ernie, its a used regulator, maybe its no good! I have described how to check it...

Share this post


Link to post
Share on other sites

Well ernie,,,,,,  were still waiting,,,,,,,,,,,,,,,,,,,,,,,how the milkcart,,,,,,,,,,,,,,:rolleyes:

Share this post


Link to post
Share on other sites

It were the fastest in the West

  • Haha 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Regiser for membership in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×